1. Field
The invention is in the field of methods and equipment for producing directed, high-velocity streams of compressible fluids, particularly useful in operating gas turbines.
2. State of the Art
In present turbines, the Brayton cycle is used to generate directed, high velocity streams of compressible fluids. This cycle consists of adiabatic compression of the fluid in which the enthalpy is increased by mechanical work; constant pressure heating that further increases the enthalpy of the fluid; and adiabatic expansion by which a portion of the enthalpy of the fluid is converted to velocity. The heating is usually obtained by combustion of fuel with the fluid.
The Brayton cycle has a disadvantage in that, when heat is added to the fluid without a corresponding increase in pressure before entering the expansion part of the cycle, a substantial part of the added heat becomes unavailable for conversion to kinetic energy. This is because the nozzle concerned is dependant, for its ability to expand the fluid and thereby convert its thermal energy into kinetic energy, on a commensurate pressure differential between inlet and outlet ends of the nozzle. In the Brayton cycle, the pressure at the inlet of the nozzle is normally that of the compressed fluid being supplied by the compressor. Any addition of heat within the nozzle that increases the pressure requires a corresponding increase in the work of the compressor and consequently offsets the advantage otherwise gained by the addition of heat. Therefore, in the Brayton cycle, heat is added in such a way as to insure that the pressure remains constant.
Adiabatic compression is thermodynamically a reversible process, i.e. isentropic, and the compressor work is essentially all recoverable as equivalent kinetic energy during expansion of the compressed fluid in a suitable nozzle. The same is not true for thermal energy added to the fluid at constant pressure. Such energy addition is largely an irreversible process and the main cause of low thermodynamic efficiencies in current methods of producing directed high velocity streams of compressible fluids. In those cases where pressure ratios would require convergent-divergent nozzle types for complete expansion of the fluid, the recoverable portion of the enthalpy of the fluid is due almost entirely to compressor work that is non-productive so far as end results are concerned. The productive portion of the fluid cycle comes from the limited advantage that can be gained by raising the temperature and by thereby increasing the critical or acoustic velocity which is recoverable in nozzles of the convergent type. This has led to the exclusive use of convergent type nozzles in current gas turbines and jet engines and a trend toward ever higher temperatures in order to obtain higher velocities and more favorable ratios between productive and non-productive work. The heat units required to secure a given velocity in this way, however, are always more than those required to obtain equivalent kinetic energy, because the higher the velocity that can be obtained the higher the exhaust temperature and consequently the greater the the heat loss. It is common in present gas turbines for the energy lost to be twice as much as the energy converted to flow velocity. Various devices are employed to recover as much of this waste heat as possible, but such devices are complicated, costly, and generally capable of recovering only a small part of the lost thermal energy.
In contrast to the Brayton cycle, steam turbines employ the Rankine cycle wherein steam is heated in a closed vessel. In this way, pressure is increased along with temperature. The process compares to that of adiabatic compression in gas turbines and consequently is essentially isentropic and therefore thermodynamically reversible. Most of the thermal energy added is recoverable as kinetic energy in suitably designed nozzles. However, because a vapor cycle is used, the exit steam still contains large amounts of heat of vaporization, which is lost in the condensers as the spent steam gives up its heat to whatever cooling medium is used. Generally, the quantity of heat absorbed in the condensers is two or three times as much as that converted to kinetic energy in the rest of the turbine cycle.